1. Field of the Invention
The present invention relates to work stations or workholders that are employed in automatic wire bonders to clamp semiconductor devices that are to be wire bonded. More particularly, the present invention relates to a novel method of bonding fine wires to the fingers or ends of leads of a lead frame to complete the second bond of a fine wire interconnection on a lead frame positioned in a quick change workholder.
2. Description of the Prior Art
The present invention relates to a substantial improvement in the method of making second bonds on the ends of leads of a lead frame held in a workholder. The prior art workholders or work stations are typical of the work station shown and described in U.S. Pat. No. 4,765,531 issued Aug. 23, 1988. This prior art improvement in workholders enabled automatic wire bonders made by Kulicke and Soffa Industries of Willow Grove, PA to consistently and uniformly make first and second bonds on semiconductor devices and such carriers as stamped lead frames. The prior art workholder has been successfully employed on automatic wire bonders designated Model 1482 to 1484 and variations thereof which are adapted to use the above mentioned quick change workholder.
Heretofore such quick change workholders of the type shown in U.S. Pat. No. 4,765,531 have employed removable or replaceable modules that are adapted to be quickly removed from the bonding machine. The removable or replaceable modules were custom designed for the many different types of semiconductor device lead frames to be bonded so as to provide precise locating and firm clamping of the ends or the fingers of the lead frames. The ends of the fingers are clamped between a lower movable heat block and a fixed and precisely planerized top plate.
Heretofore the preferred mode of wire bonding fine wire interconnections between the pads on the semiconductor device at a first bond and the ends of the leads or fingers of the lead frames at the second bond was to assure that the semiconductor and all of the ends of the leads to be bonded on the lead frame were held tight against the movable heat block which served as a mass so as to form a fixed and unyieldable mandrel so that the semiconductor and the ends of the fingers of the lead frame to be bonded were firmly clamped. To achieve the desirable type clamping of the fingers of the lead frame it was common practice in the prior art to clamp the leads as close to the ends as possible and to attempt to force the ends of the leads or fingers into engagement with the heat block to assure that the finger was tightly clamped so as to prevent any looseness and to transfer heat from the heat block to the ends of the fingers to be bonded. Proper thermosonic wire bonds at first and second bonding sites are a function of time, force and ultrasonic power applied through the bonding tool to the wire being bonded. Heating the semiconductor device pads and/or the lead frame fingers is known to reduce the bonding time. Alternatively the raising of the temperature of the bonding target has been employed to reduce the required bonding force to make high yield strong bonds. It is known that the application of an ultrasonic scrub motion to the bonding tool during first and second bond further reduces the bonding time and/or may be used to reduce the bonding force at first and second bond.
Very high lead count lead frames are used with very large scale integration (VLSI) semiconductor devices. Such high count lead frames require that the lead frame fingers be made very small to prevent touching adjacent leads and to provide the necessary number of leads for making interconnections from the lead out pads at the outer perimeter of the high density VLSI semiconductor devices. Not only have the fingers of the lead frame been made smaller but they are positioned further from the bonding targets on the semiconductor devices so that the fine wire interconnection must be made more accurately than in the prior art and the shape of the loop of the interconnecting fine wires must be more precise than was heretofore necessary.
One of the problems which has existed with prior art workholders is that they are extremely expensive and must be custom designed for the many different types and sizes of lead frames to assure proper clamping.
It would be extremely desirable to provide a method and apparatus for making fine wire interconnections on high density semiconductor devices using inexpensive clampless workholders and programmable automatic wire bonders of the type that may be reprogrammed to carry out a new and improved bonding method.